Raman spectroscopy is a very powerful tool for chemical and biochemical analysis. However, as is well known, spontaneous Raman transition spectrophotometry provides very weak signals that require long data acquisition times and high power lasers. Its performance is limited when low concentration target samples are being analyzed.
Stimulated Raman scattering (SRS) spectroscopy is an advanced spectroscopic technique, based on the Raman phenomena, useful to probe, “fingerprint” and quantitatively determine the concentration of target molecules. SRS amplifies the Raman phenomena by activating two monochromatic laser beams on a sample being investigated, a Stokes laser beam with intensity Is and a pump laser beam with intensity Ip. When the frequency difference of the two beams Δω=ωp−ωs=Ω (ωp and ωs signifying the pump beam and Stokes beam frequencies, respectively) matches the natural frequency of vibration of a target molecule Ω or vibrational-rotational mode of a target molecule, stimulated excitation of a Raman mode transition occurs in the target molecule. Stimulated Raman scattering (SRS) spectroscopy provides very good performance with short acquisition times and low average power.
When the condition Ω=Δω is met, the intensity of the pump field experiences a loss ΔIp (SRL) while the Stokes field experiences a gain ΔIs (SRG). The gain changes of the Raman scattering process of the beams (SRL or SRG) are proportional to the quantity of the target molecule in the sample. In a small signal regime—when the intensity of SRG or SRL is small, i.e., ΔI/I<<1, ΔIs and ΔIp, are described by:ΔIs∝N×σRaman×Ip×Is  [Eq 1]ΔIp∝−N×σRaman×Ip×Is  [Eq 2]where ΔI refers to the change in intensity I of the pump and Stokes laser beams, ΔIp and ΔIS respectively, where I is the intensity of the pump and Stokes laser beams Ip, and IS, respectively; N is the number of molecules in the probed/tested volume, and σRaman is the molecular Raman scattering cross-section.
In order to achieve high resolution molecular measurements where the number of molecules in the probed volume is very low, it is clear from Eq. 1 that Ip× IS should be very high, i.e. the irradiances (intensities) (W/cm2) of the pump and the Stokes laser, Ip and Is respectively, should be very high. However, the noise in the system is also proportional to Ip× Is. Therefore, in cases of very low concentration where ΔI<<<I, the signal to noise ratio (SNR) denoted by ΔI/n (n being noise) is very low.
From the above, it is readily understood by persons skilled in the art that the challenges that must be dealt with when using a SRS spectrophotometer system in the low concentration regime are low signal to noise ratio (SNR) and poor analog-to-digital conversion, i.e., the measured signal (I) may be high while the relevant signal (ΔI) is very low.
As noted above, in the SRS technique, the gain of Raman scattering is proportional to the electro-optical field amplitude of the pump and the Stokes beams. In addition, the results are highly dependent on the accuracy of the wavelength difference of the beams. In order to achieve high resolution spectral measurement of target materials (i. e. target molecules), prior art SRS spectroscopy systems use high peak power, and femtosecond and/or picosecond lasers. When laser intensity is low, the beam diameter is reduced to maintain a minimum of about 10 MW/cm2 needed for each SRS laser beam. Wide range tunable lasers are used to acquire a wide range Raman spectrum. A high level of wavelength and amplitude stabilization is required. In addition, very fast, high resolution photodetectors and real time noise reduction techniques have been used.
All this leads to very complicated implementation, which is suitable only for university laboratories and research institutes.
SRS is typically implemented in the near infra-red (NIR) region of the electromagnetic spectrum (600-1000 nm) where other physical spectrometric phenomena, e.g., fluorescence, have low expression and the molecular Raman scattering cross-section (σRaman) is high. These two factors result in a high “built-in” signal-to-noise ratio. Of particular importance is water (H2O) which is present in most materials and has virtually no fluorescence in the NIR region indicated above.
Current solutions for managing the low SNR and poor resolution of SRS systems require using high peak power, narrow spectral emission widths, very accurate and stable (low noise) optical components (laser sources, photodiodes, etc.) and high resolution, low noise analog-to-digital converters (ADCs). However, these components are expensive and in many cases must be custom made. Moreover, the system's architecture is overly complicated, bulky, relatively delicate, and difficult to align and maintain alignment. It also does not allow for outdoor use. Alternatively, there are lasers which are less accurate, inherently unstable, express high background, subject to wavelength drift, and are hereby defined as unstable and possessing large impairment. Devising a method of using such unstable lasers in SRS could significantly lower cost, instrument size and increase system robustness.